Thermal spray compositions for abradable seals

ABSTRACT

A thermal spray composition and method of deposition for abradable seals for use in gas turbine engines, turbochargers and steam turbines. The thermal spray composition includes a solid lubricant and a ceramic preferably comprising 5 to 60 wt % total of the composition in a ratio of 1:7 to 20:1 of solid lubricant to ceramic, the balance a matrix-forming metal alloy selected from Ni, Co, Cu, Fe and Al and combinations and alloys thereof. The solid lubricant is at least one of hexagonal boron nitride, graphite, calcium fluoride, lithium fluoride, magnesium fluoride, barium fluoride, tungsten disulfide and molybdenum disulfide particles. The ceramic includes at least one of albite, illite, quartz and alumina-silica.

This application is a Divisional of application Ser. No. 10/164,009filed Jun. 7, 2002, now U.S. Pat. No. 6,887,530.

BACKGROUND OF THE INVENTION

(i) Field of the Invention

This invention relates to abradable seals and, more particularly,relates to high-temperature abradable seal compositions for use in gasturbine engines, turbochargers and steam turbines, and their method ofdeposition.

(ii) Description of the Related Art

Basic requirements for abradable seals in the compressor section of gasturbine engines include good abradability, spall resistance, and erosionresistance. Abradable seals are also required to exhibit low gaspermeability, a smooth surface, good aging properties and long termoxidation resistance at high temperatures. In the case of abradability,the seal is a sacrificial element, it being desirable to minimize bladewear. Additionally, low gas permeability is required in order tominimize gas flow through the seal itself. It has been shown that lowpermeable seals with a smooth surface finish improve overall compressorefficiency by about one percent as compared to conventional permeableseals. In addition, low permeability of the seal prevents entrapment offine particles, e.g. dust or grit, which can act as an abrasive againstthe blade tips, thus wearing them unevenly. Smooth surface finishes inthe gas path improve overall airflow, also contributing to efficiency.Finally, long-term oxidation resistance is required due to increases incompressor operating temperature up to 815° C.

There are several air seals used in a compressor section of a gas oraircraft engine. Historically the oldest is feltmetal that comprises aplurality of metal fibres. The feltmetal is described for example inU.S. Pat. No. 4,257,735. The most important disadvantages of this sealare that it has to be brazed to the substrate material and that it ishighly porous.

Typical jet engine compressor air seals include a metal matrix ofaluminum and silicon with embedded polymer particles or hexagonal boronnitride powder particles as described in U.S. Pat. No. 3,723,165 and No.5,506,055, respectively. The disadvantage of these systems is theirlimited temperature capability at 315° C. for the system with polymerand 480° C. for the system with hexagonal boron nitride. In the formercase, the temperature capability is governed by the polymer and in thelatter case it is governed by the aluminum silicon alloy.

Abradable materials used at high temperatures in the compressor sectionof turbine engines are usually NiCrAl/Bentonite coatings. However,NiCrAl/Bentonite seals do not rub well against Ti alloy blades. Thesecoatings perform well against Ni alloy and steel blades but, when Tialloy blades are used, the blade tips overheat and are subject to wear.Sometimes, glazing of the coating is observed.

Another known abradable seal is that prepared by the techniques ofRangaswamy et al., described in U.S. Pat. No. 5,434,210. A compositepowder for thermal spraying of abradable coatings is disclosed in whichthe composite powder contains three components. One component is any ofa number of metal or ceramic matrix materials, another component is asolid lubricant, and the third component is a polymer. Typicalas-sprayed coatings comprise a Co alloy matrix with dispersed particlesof hexagonal boron nitride and polymer. The polymer is subsequentlyburned out and the final very porous structure contains only hexagonalboron nitride particles dispersed throughout the Co-based matrix. Thecoatings prepared from this material have acceptable abradability butlow erosion resistance and deposition efficiency. The erosion resistanceis required in order to maintain uniform clearances throughout the lifeof the engine or performance characteristics are adversely affected.Conventional commercial turbine engines have exhibited a two percentincrease in airflow around blade tips as a result of seal erosion afterapproximately 3,000 flights. Much of this may be attributed to erosionof the abradable seal and blade airfoil tip, and to rub interactionsbetween the blade tips and the seal. In military engine applications,where gas path velocities are relatively high, erosion resistance is ofparamount importance.

It is accordingly a principal object of the present invention to providean abradable seal, for use in gas turbine engines at temperatures up toabout 815° C., having good abradability, spall resistance and erosionresistance, particularly when used in conjunction with titanium-alloyblades.

It is another object of the present invention to provide an abradableseal composition having a smooth surface, low permeability and long-termoxidation resistance resulting in favourable long-term agingcharacteristics.

A further object of the invention is the provision of a novel thermalspray material and its method of application for producing an abradableseal.

SUMMARY OF THE INVENTION

In its broad aspect, the three-phase thermal spray composition of theinvention for an abradable seal comprises a mixture of at least 5 wt %total, preferably 19 to 60 wt % and more preferably 25 to 45 wt %, of asolid lubricant and a ceramic, the balance of a matrix-forming metalalloy. The composition can be in the form of a wire, powder or rod. Thesolid lubricant is at least one of hexagonal boron nitride, graphite,calcium fluoride, lithium fluoride, magnesium fluoride, barium fluoride,tungsten disulfide and molybdenum disulphide particles, preferablyhexagonal boron nitride powder. The ceramic preferably is at least oneof albite, illite and quartz particles or mixture thereof, andalumina-silica particles. The ratio of solid lubricant to ceramic is 1:7to 20:1, preferably 1:6 to 9:1. The matrix-forming metal alloy isselected from Ni, Co, Cu, Fe, Al and combinations and alloys thereof,particularly NiCrAl, NiCr, CuAl and AlSi. Other elements such as Y, Hf,Si, Nb, Re and Ta can be added in small amounts to increase oxidationresistance of the matrix-forming metal alloy. The matrix-forming metalalloy can also contain some other elements as impurities thatsignificantly do not alter alloy properties. The three-phase thermalspray composition preferably comprises the matrix-forming metal alloycoating at least one of the ceramic particles and hexagonal boronnitride particles. The thermal spray composition can include aparticulate fugitive material such as a consumable polymer in an amountof 1 to about 30 wt %/o of the composition. Another preferredcombination of materials is hexagonal boron nitride andalumina-silica-based ceramics in a total amount of 10 to 50 wt %, thebalance Ni alloy. Salt, sugar and other fugitive materials would servethe same purpose as the polymer, i.e. to create porosity after theirelimination from the abradable coating.

In accordance with another broad aspect of the invention, the abradableseal consists essentially of at least 5 wt % total of the solidlubricant and the ceramic, preferably 19 to 60 wt % and more preferably25 to 45 wt % total of the solid lubricant and ceramic, in a ratio of1:7 to 20:1 of solid lubricant to ceramic, the balance a matrix-formingmetal alloy selected from the group consisting of Ni, Co, Cu, Fe and Aland combinations and alloys thereof, preferably NiCrAl, NiCr, CuAl orAlSi.

The method of providing an abradable seal on a substrate comprisesapplying an adherent coating of the said three-phase composition havinga thickness of up to 3 mm onto the substrate by thermally spraying apowder, wire or rod composition thereon.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The abradable seal of the present invention comprises a matrix-formingmetal alloy component, a solid lubricant component and a ceramiccomponent, wherein the three components provide a synergism in abradablecoatings which have unexpected superior characteristics over prior artmaterials. The matrix-forming metal alloy is selected from the metalsNi, Co, Cu, Fe and Al and combinations and alloys thereof, preferablyNiCrAl, NiCr, CuAI and AlSi. The ceramic component is typified by oxidicceramics such as ceramics in the group comprising albite, illite andquartz and mixtures thereof and preferably is clad with a metal alloy bya hydrometallurgical process well known in the art prior to blendingwith particles of a lubricant such as hexagonal boron nitride, graphite,calcium fluoride, lithium fluoride and molybdenum disulphide particles,preferably hexagonal boron nitride. A preferred combination of metalalloy or oxidic ceramic is at least one of or a mixture of albite,illite and quartz clad with a NiCrAl coating on the ceramic. A materialfound to be satisfactory is commercially available as Durabrade™ 2313from Westaim Ambeon. Another preferred combination of materials ishexagonal boron nitride and alumina-silica-based ceramics in a totalamount of 10 to 50 wt %, the balance nickel alloy. The clad ceramicparticles can be blended with particles of hexagonal boron nitrideavailable from commercial suppliers.

The ceramic and lubricant components together comprise at least 5 wt %and not more than 60 wt % of the composition, preferably 19 to 60 wt %and more preferably about 25 to 45 wt % of the composition, the balancecomprising the matrix metal material. The ratio of solid lubricant toceramic is 1:7 to 20:1, preferably 1:6 to 9:1.

Although the description proceeds herein with reference to a blend ofclad ceramic particles and hexagonal boron nitride, it will beunderstood that the composition may also be prepared in the form ofmechanical blends of the three components, homogenously agglomeratedparticles, mechanically fused particles, agglomerated ceramic and solidlubricant particles clad with metal alloy or mixtures of metal cladceramic and metal clad solid lubricant, or as a continuous solid orcored wire.

The three-phase composition of the invention preferably is applied to asubstrate by thermal spray to form an abradable seal. Thermal sprayinginvolves the softening or melting of a heat fusible material such asmetal or ceramic by heat, and propelling the softened or melted materialin particulate form against a surface to be coated. The heated particlesstrike the surface where they are cooled and bonded thereto. Aconventional thermal spray gun may be used for the purpose of the bothheating and propelling the particles.

A thermal spray gun normally utilizes a combustion or plasma or electricarc to produce the heat for melting of the composition in either powderor wire form. In a powder type combustion thermal spray gun, the carriergas, which entrains and transports the powder, is typically an inert gassuch as argon. In a plasma spray gun, the primary plasma gas isgenerally argon or nitrogen. Hydrogen or helium is usually added to theprimary plasma gas, and the carrier gas is generally the same as theprimary plasma gas. Other thermal spray methods could also be used. Agood general description of thermal spraying is provided in U.S. Pat.No. 5,049,450. The matrix-forming metal alloy powder such as NiCrAl canbe dry blended and mixed with the ceramic powder and the lubricantpowder, at least one of the ceramic powders and the lubricant powderspreferably having a coating of the matrix-forming metal alloy thereon.The matrix metal alloy holds the particles of the ceramic and the solidlubricant in place and adherently bonds the coating to the substrate.

The resulting product is a homogenous abradable seal material comprisingthe continuous metallic matrix phase which provides structural strengthto the composition up to at least an operating temperature of 815° C.The solid lubricant decreases the hardness and strength of the metallicmatrix material due to its uniformly dispersed presence as solid softlubricant particles to enhance ejection of the particles from thecoating when abraded. Due to the softness of the lubricant, blade wearis decreased and abradability of the coating is improved. The solidlubricant occupies pore spaces between the metal matrix and ceramicparticles to provide a dense structure which will function as a heatsink having enhanced thermal conductivity to conduct heat from the rubzone of the seal. The ceramic component provides enhanced erosionresistance while contributing to the decrease in matrix strength withimproved abradability. During the rubbing process by the titanium alloycompressor blades, the seal material abrades readily due to low matrixstrength caused by the synergistic effect of the solid lubricant and theceramic materials. The metal alloy matrix would collapse and compressduring the rubbing process without the presence of the ceramic and solidlubricant filler. The coating would become very hard and would glazewith subsequent significantly decreased abradability. Coating glazingthus is limiting by facile particle ejection when rubbed by a compressorblade while acceptable erosion resistance is maintained by the additionof the ceramic material.

The solid lubricant and ceramic particles are uniformly dispersedthroughout the deposited coating. Thus any material removal that occursdoes not change coating properties of the remaining seal. Coatings ofthe invention prepared by thermal spraying the coating composition ontoa substrate have an excellent combination of abradability and erosionresistance as a result of the coating properties including low coatingcohesive strength, low porosity, low surface roughness and high thermalconductivity. As a result, the coatings can be effectively used incombination with titanium-alloy blades without danger of titanium fire,coating glazing or excessive blade wear. Overall engine efficiency isincreased.

If it should be desired to increase porosity of the abradable seal, aparticulate polymer having a size range of 1 to 200 μm can be blendedwith the three-phase composition material in an amount of 1 to 30 wt %of the composition prior to thermal spraying. The fugitive polymer isvaporized, burned out or leached to yield a porous and permeablecoating. A list of the polymers which can be used is in U.S. Pat. No.4,917,960, incorporated herein by reference.

The abradable seal of the invention will now be described with referenceto the following non-limitative example.

EXAMPLE

Commercially available Durabrade™ 2313 powder produced by Westaim Ambeonhaving a particle size in the range of −100+200 mesh was blended with 15wt % of hexagonal boron nitride lubricant having a particle size of−100+200 mesh sold by Advanced Ceramic Corp. under the name AC 6001. Thetotal of the ceramic and lubricant particles comprised 32 wt % of thepowder composition, the balance NiCrAl alloy. The powder blend was flamesprayed (oxygen-acetylene mixture) onto a substrate and the resultingcoating properties were compared with the Co-alloy based seal describedin U.S. Pat. No. 5,434,210 and sold as SM 2042 by Sulzer Metco:

Applicants' Co-based Seal Seal (U.S. Pat. No. 5,434,210) Density [g/cm³]3.4 2.68 Hardness (HR15Y) 19 40 Ambeon Erosion 90° [g/min] 0.5 2.05Ambeon Erosion 90° 147 765 [mm³/min] Coating Strength [psi] 299 788Deposition Efficiency [%] 47 N/A Thickness [mm] 2 2 Abradability (bladewear [inch]) 1000-0.0002-0.03* 0.001 0.003 820-0.02-0.03 0.008 0.026*Abradability was measured using a Ti alloy test blade that rubbedagainst tested seals. Test conditions were: 1000 and 820 ft/s blade tipspeed, 0.0002 and 0.02 in/s incursion rate and 0.03 inch totalincursion. All rub paths of the seal of this invention were smooth withno glazing or discoloration.

The particles of the ceramic and the boron nitride were uniformlydispersed throughout the coating. The good properties of the applicants'seal are demonstrated by substantially-improved erosion resistance andabradability performance compared to the Co-based seal.

It will be understood, of course, that modifications can be made in theembodiments of the invention illustrated and described herein withoutdeparting from the scope and purview of the invention as defined by theappended claims.

1. A thermal spray composition comprising 5 to 60 wt % total of a solidlubricant and a ceramic, the balance a matrix-forming metal alloywherein the solid lubricant is hexagonal boron nitride, the ceramic isat least one ceramic selected from the group consisting of albite,illite, mixtures thereof and alumina-silica-based ceramics in a ratio of1:7 to 20:1 by weight of solid lubricant to ceramic, and thematrix-forming metal alloy is an alloy selected from the groupconsisting of alloys of Ni, Co, Fe and combinations thereof.
 2. Athermal spray composition as claimed in claim 1, additionally comprisinga particulate consumable polymer comprising 1 to about 30 wt % of thecomposition.
 3. A thermal spray composition as claimed in claim 4, inwhich the ceramic includes at least one of albite and illite or mixtureThereof.
 4. A thermal spray composition as claimed in claim 1, in whichthe thermal spray composition is a wire, a powder or a rod.
 5. A thermalspray composition for an abradable seal as claimed in claim 4 comprising5 to 30 wt % total of hexagonal boron nitride and ceramic.
 6. A thermalspray composition as claimed in claim 4, in which the matrix-formingmetal alloy is Ni alloy.
 7. A thermal spray composition as claimed inclaim 6, in which the ceramic includes at least one of albite and illiteor mixture thereof.
 8. A thermal spray composition as claimed in claim7, in which the total of solid lubricant and ceramic is about 25 toabout 45 wt % of the composition.
 9. A thermal spray composition asclaimed in claim 8, in which the solid lubricant and ceramic are presentin a ratio of 1:6 to 9:1 by weight of solid lubricant to ceramic.
 10. Athermal spray composition as claimed in claim 7, in which thematrix-fanning metal alloy is Ni alloy, said Ni alloy coating at leastone of the ceramic particles and the hexagonal boron nitride particles.11. A thermal spray composition as claimed in claim 7, additionallycomprising a particulate consumable polymer comprising 1 to about 30 wt% of the composition.
 12. A thermal spray composition as claimed inclaim 6, in which the ceramic is an alumina-silica-based ceramic.
 13. Athermal spray composition as claimed in claim 12, in which the total ofsolid lubricant and ceramic is about 10 to about 45 wt % of thecomposition.
 14. A thermal spray composition as claimed in claim 13, inwhich the solid lubricant and ceramic are present in a ratio of 1:6 to9:1 by weight of solid lubricant to ceramic.
 15. A thermal spraycomposition as claimed in claim 12, additionally comprising aparticulate consumable polymer comprising 1 to about 30 wt % of thecomposition.
 16. A thermal spray composition as claimed in claim 6, inwhich the matrix-forming metal alloy is NiCrAl.
 17. A thermal spraycomposition as claimed in claim 16, in which the ceramic includes atleast one of albite and jute or mixtures thereof.
 18. A thermal spraycomposition as claimed in claim 16, in which the total of solidlubricant and ceramic is about 25 to about 45 wt % of the composition.19. A thermal spray composition as claimed in claim 18, in which thesolid lubricant and ceramic are present in a ratio of 1:6 to 9:1 byweight of solid lubricant to ceramic.